Makefile: correct example fuzz build

The comment explaining how to build the fuzzers was broken in
927c77e7d4 ("Makefile: use FUZZ_CXXFLAGS for linking fuzzers",
2018-11-14).

When building fuzzers, all .c files must be compiled with coverage
tracing enabled. This is not possible when using only FUZZ_CXXFLAGS, as
that flag is only applied to the fuzzers themselves. Switching back to
CFLAGS fixes the issue.

Signed-off-by: Josh Steadmon <steadmon@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Makefile

@@ -3104,7 +3104,7 @@ cover_db_html: cover_db
 # An example command to build against libFuzzer from LLVM 4.0.0:
 #
 # make CC=clang CXX=clang++ \
-#      FUZZ_CXXFLAGS="-fsanitize-coverage=trace-pc-guard -fsanitize=address" \
+#      CFLAGS="-fsanitize-coverage=trace-pc-guard -fsanitize=address" \
 #      LIB_FUZZING_ENGINE=/usr/lib/llvm-4.0/lib/libFuzzer.a \
 #      fuzz-all
 #